(1) Field of the Invention
The present invention relates to a board for an additive circuit which is suitable for electric circuits used in electronic apparatuses.
(2) Description of the Prior Art
Additive circuits of this type are formed by precipitating a conductive pattern on a substrate board by using electroless disposition alone or in combination with an electro-plating method which does not require cladding of copper in advance on the surface of an insulating material. The surface of a board for additive circuits is required to have plural minute convexo-concave irregularities in order to improve bonding with plated layers.
In prior art, organic composite materials such as epoxy/glass laminated plates are often used as the material for such boards because of advantages in producting them and their mechanical workability. However, the laminated plate must be treated with chromic acid to roughen the surface in order to improve bonding in electroless deposition, thereby making the process complicated and pushing up the manufacturing cost due to the expensive treatment needed for waste disposal. There have been proposed various methods to use resin on the surface of such laminated plates which are expected to promote roughening for a higher bonding in plating. The resin provided on the surface of the laminated plate, however, makes the laminated plate susceptible to humidity.
The above organic composite materials, especially the organic component such as the adhesive therein are inferior in heat resistance. As they therefore do not have the desired insulation resistance at high temperatures nor good dielectric characteristics and are susceptible to humidity, they are detrimental in that they cannot be used at higher temperatures for a long time.
As the general trend in electronic apparatuses aims at minimizing the size and decreasing the thickness of a device, the packaging density of parts on a substrate for a printed circuit board is increasing while such substrate is required to have excellent radiation and size stability characteristics.
The copending application referred to above based on Japanese priority applications No. Sho 57-78028 and 57-78029 for "An Inorganic Composite and the Preparation Thereof" which is characterized in that a needle-like or fiber-like inorganic material is present as a mixture in an isotropic solid vitreous inorganic material while maintaining its needle-like or fiber-like form and which is prepared by a method comprising the steps of kneading a mixture containing a vitreous inorganic material and a needle-like or fiber-like inorganic material, molding the mixture into a predetermined shape, calcining the molded material at a temperature above the softening point of the vitreous inorganic material and below the melting point of the needle-like or fiber-like inorganic material is designed to obviate the defects of the prior art substrates.
As the above inorganic composite is molded into a flat plate by extrusion or compression molding, at least one of the molded surfaces tends to form a smooth surface. If such plate is used as the board for additive circuits without further treatment, the adhesion with the plated layer becomes unstable.